In general, carbon fiber is used in composite enhanced substances with different types of substrates, and the adhesiveness of a specific substrate is important to carbon the fiber to show the characteristics of the enhanced substances.
Non-surface treatment carbon fibers usually have insufficient adhesiveness for the substrate and poor transverse properties such as the separating strength and shear strength. Naturally, the carbon fiber generally receives an oxidization treatment such as an electrolytic oxidation treatment, a gas-phase chemical oxidation treatment or a liquid-phase chemical oxidation treatment after a carbonization or graphitization takes place, and oxygen-containing functional groups are added into the carbon fiber to improve the wettability of the substrates.
With regard to the surface characteristics of the carbon fiber after being processed by the oxidization treatment, Japan Patent Publication 4-361619 disclosed a method of improving the adhesive strength of the carbon fiber to the substrate by depositing a specific functional group onto the uppermost surface of the carbon fiber. This patent also specifies the carbon fiber whose surface oxygen concentration and surface nitrogen concentration are both measured by X-ray optoelectronic spectroscopy measurement techniques (such as those disclosed in Japan Examined Patent Publication No. 4-44016, and Japan Unexamined Patent Application Publication Nos. 2-210059, 2-169763, 63-85167, and 62-276075), but these inventions do not include the study of a combination of a sizing agent. In addition, these patents are simply described by using the surface functional groups such as the drawback of having a poor adhesion with a substrate, particularly the substrate with a low reactivity.
On the other hand, carbon fiber and graphite fiber are peculiarly hard and brittle and lack of adhesiveness, bending strength and wear-resistance, so that various different types of sizing agents are generally added into the treated carbon fiber to prevent the formation of fine hair and the occurrence of thread fracture, so as to improve the adhesiveness, bending strength and wear-resistance. Studies on developing and using a sizing agent such as a paste or an adhesive to improve the treatment are conducted, but the study on the improvement of adhesiveness of the sizing agent to a substrate has not been actually performed. In addition, the study on modifying surface characteristics by a sizing agent has not been performed. For example, the functional groups on the aforementioned carbon fiber surface are used to improve the overall characteristics including adhesiveness and tension of a composite.
Since the most popular substrate used in a carbon fiber enhancing composite substance is epoxy resin, and the sizing agent is usually epoxy resin or modified epoxy resin which are biphenol A diglycidyl ether type epoxy resins such as aromatic compounds with a structure related to the substrate (as disclosed in Japan Examined Patent Publication No. 4-8542, Japan Unexamined Patent Application Publication No. 1-272867, and Japan Examined Patent Publication Nos. 62-56266 and 57-15229). However, the aforementioned common sizing agent is a thermosetting resin, so that if it is necessary to produce a thermoplastic carbon fiber composite material, then the interface will not be matched, and the carbon fiber and the resin will be unable to form a complete joint interface, and the sizing agent cannot be used extensively for electric/electronic components, mechanical components or automobile components which are manufactured by injection molding. Therefore, it is an urgent and important subject for related manufacturers to develop a changing method capable of changing the carbon fiber surface oil.